The invention concerns a device for measuring the extension of a threaded bolt or screw tightened by means of a nut.
To apply a specific preload to mounting screws used in mechanical engineering, these screws and/or nuts are tightened mainly with torque wrenches.
These wrenches may be operated manually, driven electrically, or hydraulically. With torque wrenches as tightening devices, it is disadvantageous that no definite relation can be made between the torque applied and the tension of the bolt and/or screw actually achieved. This is caused by the varying coefficients of friction within the thread and between the contact areas of the screw head and/or the nut used for tightening the screw or threaded bolt.
To determine the exact preload of the screw, it is necessary to measure of the achieved screw extension that is directly proportional to the applied preload. For exact measurements of the actual extension of the screw, both screw ends have to be accessible for measuring. If only one screw end is accessible for the measurement, the extension can only be measured relative to the nut contact area, resulting in inaccuracies that are caused by seating effects in the contact area and by compression of the flanges to be tightened by means of the screw. The extension can only be measured when the screw end is accessible to a measuring device, e.g., a dial gauge. Generally, the screw tightening device must be removed before the extension can be measured.
In the German petty patent document G 94 08 009.7, a ratchet wrench for angle tightening with visual and audible indication is described, wherein the screw, in particular, of a screw connection is first fastened to only a small torque with a conventionally known torque wrench in order to apply the desired preload to the shanks of screws inserted into the work piece, in particular, reduced-shaft screws. The extension of the screws in the ductile range is then achieved by further tightening the screws by a specific rotation angle. For this, the tensioning tool comprises a disk with an angle measuring scale coaxially positioned to the rotational axis of a securing attachment and indicator marks cooperating with the angle measuring scale. The disk comprising the angle measuring scale and the indicator marks are rotatable against each other coaxially to the rotational axis of the securing attachment, lock-connected with the tensioning tool on the one hand, and, on the other hand, lockably mounted to the work piece to prevent rotation. This fixation is provided by a flexible shape-retaining bar comprised of a wire spiral with a magnet head on its free end engaging the work piece. Between the securing attachment and the lever arm for rotating the securing attachment, a ratchet is provided for rotating the securing attachment in several strokes until the final rotation angle is achieved.
The object of the invention is to provide a device for measuring the extension of a threaded bolt or screw that is fast and easy to use and, in particular, during tightening of the nut or screw, allows a reading of the extension. The measurement should at least be as exact as a measurement with a dial gauge and should significantly exceed the accuracy that can be achieved with a torque wrench.